The present invention relates to an on-off controlled oscillating circuit which is suitable for achieving a power saving function and a clock stop standby function for LSI (large scale integrated circuit).
Oscillating circuits are commonly known which comprise a combination of a quartz oscillator and an amplifier circuit. In some of the oscillating circuits of this type, the oscillation operation is on-off controlled by controlling the output signal from the amplifier circuit. For example, from Japanese Patent Laid-open Publication No. HEI 6-29743, there is known an oscillating circuit which comprises a two-input NAND gate, a quartz oscillator and feedback resistor connected between one input terminal and output terminal of the NAND gate, and resonance capacitors connected to the one input and output terminals, respectively, of the NAND gate and in which the other input terminal of the NAND gate functions as a control terminal.
In the above-mentioned oscillating circuit with a control terminal, when the control signal applied to the control terminal is at a high level, the NAND gate functions as an inverter so that the oscillating circuit operates as a voltage amplifying circuit to provide oscillatory output. Once the control signal turns to a low level, the output of the NAND gate is fixed at a high level to turn OFF or deactivate the oscillation operation. In generating clocks of very low voltage under low supply voltage, a considerable amount of time is required before the oscillation operation is stabilized after the control signal having been switched to the high level to turn ON or activate the oscillating circuit. The disclosure in the above-mentioned Japanese publication is directed to how to reduce the time necessary for the oscillation operation to be stabilized after activation of the oscillating circuit.
However, the prior art on-off controlled oscillating circuits have several inconveniences such as unwanted glitch occurring when the oscillation operation is turned OFF and during a transitional period before the oscillation operation is stabilized after having been turned ON, and unbalanced duty ratio. These inconveniences would unavoidably result in unreliable operation where control is performed to deactivate the oscillating circuit in a certain operational condition and then to again activate the oscillating circuit in that condition for performing power control of LSI or monitoring LSI being processed.